Required columns on tables From 4.1.4.1 down to 4.1.5
Add missing table heading on Interaction components
fixes in response to @andyjohnson comments

fixes https://github.com/adlnet/xAPI-Spec/pull/424#discussion_r8100299 , https://github.com/adlnet/xAPI-Spec/pull/424#discussion_r8100339 , https://github.com/adlnet/xAPI-Spec/pull/424#discussion_r8100345
Addresses https://github.com/adlnet/xAPI-Spec/pull/424#discussion_r6925553
All properties on context are optional

Also added some missing </td> tags.
Change yes/no to Required/Optional for attachments
section 4.2 required/optional on tables
section 5.4 required/optional on tables
section 7 required/optional columns on tables
Remove appendices A and B (see https://github.com/adlnet/xAPI-Spec/issues/427)
Fix contents links (https://github.com/adlnet/xAPI-Spec/issues/431)
Merge pull request #424 from garemoko/morerequiredtables

Add optional/required for even more tables
Merge pull request #434 from garemoko/requiredcols_contextonwards

Add optional/required for the last set of tables
Merge pull request #435 from garemoko/remove-appendices-A-and-B

Remove appendices A and B
Merge pull request #436 from garemoko/fix-links

Fix links
fixed incorrect appendix reference
Merge pull request #443 from phamdt/conversion_typo

fixed incorrect appendix reference
Update Description of LRS Action for dupe activit.

As documented in Issue #445
Added "used" - typo
Merge pull request #446 from andyjohnson/master

Update Description of LRS Action for duplicate activities